A typical image sensor has an image sensing portion that includes a photosensitive area for collecting a charge in response to incident light. Typically, these images sensors include a number of light sensitive pixels, often arranged in a regular pattern of rows and columns. Each pixel includes a photosensor, such as a photodiode, that produces a signal corresponding to the intensity of light impinging on that pixel when an image is focused on the array.
One type of image sensor is a Complementary Metal Oxide Semiconductor (CMOS) image sensor, in which the image sensing portion includes a photodiode for collecting charge and a transfer gate for transferring the charge from the photodiode to a charge-to-voltage conversion mechanism, such as a floating diffusion. Usually, the sensing portion and the control circuitry for the image sensor are fabricated within a single material layer.
In an effort to increase the number of pixels provided in an image sensor, pixel size has been decreasing. An advantage of moving to smaller pixels is that it increases the resolution of the image for a fixed optical format. Specifically, smaller pixels have a better modulation transfer function (MTF), and can thus discriminate fine details in an image, such the lines on a finely striped shirt.
However, as pixels made using CMOS processes scale to smaller dimensions, several performance properties of the imagers using these pixels can degrade. Particularly, optical sensitivity (OS) degrades quickly. This is because both the quantum efficiency (QE) degrades with decreasing aperture size, and the pixel area also shrinks. Because OS depends on the product of QE and pixel area, OS is negatively impacted by both.
Thus, a need exists for an improved image sensor structure.